1. Field of the Invention
The present invention relates generally to a semiconductor type accelerometer and more specifically to such a type of sensor which utilizes a piezo resistance effect to determine the amount of G force being applied to a movable mass member or element thereof.
2. Description of the Prior Art
FIGS. 1 and 2 show a prior art semiconductor accelerometer disclosed in IEEE Electron Devices, Vol.ED-26, No. 12 P1911, Dec. 1979 "A Batch Fabricated Silicon Accelerometer". As shown this device consists of a monocrystalline silicon wafer or base member 1 in which an essentially rectangular shaped cut-out 2 is formed. This cut-out defines an essentially rectangular section 3 which is connected to the remainder of the base member through a bridge-like portion 4. The lower face of the base member 1 is recessed in a manner which reduces the thickness of the "bridge" portion 4 and thus renders it relatively flexible. With this configuration the rectangular section 3 defined within the cut out is able to act as a pendulum or a mass member which is responsive to the G forces applied thereto when the device is subject to acceleration.
Four pizeo resistances 5a-5d are formed in the upper surface of the base member 1. Each of these resistances are defined by P type regions. Three heavily doped P+ regions 6-8 are formed in a manner as illustrated and act as leads for establishing electrical contact between the resistances and circuitry external of the chip.
The chip is disposed within a glass package which is filled with a suitable liquid or gaseous damping fluid.
When a G force is applied to the bridge portion 4 the piezo resistances 5a, 5b formed thereon are subject to flexure. This causes the resistance values of the same to vary. By connecting the reistances in a manner to define bridge circuit and measuring either the change in voltage or current flow which is induced by the change in resistance, the amount of force (acceleration) to which the mass member is subject can be measured.
However, with this type of arrangement a drawback has been encountered in that as the surface on which the piezo resistances 5a-5d are formed is uncovered, electrical leakage tends to occur and reduces the stability with which measurement can be effected.
In order to overcome this problem it has been proposed to form a SiO2 membrane (not shown) over the surface of the chip using a planar technique. This, while obviating the leakage problem has induced a further drawback in that as the expansion coefficients of the monocrystalline silicon wafer 1 and the SiO2 membrane are different, differences in the thickness of the Si and SiO2 layers in which the "measuring" and "reference" piezo resistances 5a, 5b & 5c, 5d are respectively formed, induces the situation wherein the temperature variation with respect to the amount of flexure is different and causes a difference in temperature drift which interferes with the accuracy of the device.